A composite material formed by impregnating carbon fiber with resin as matrix is lighter and higher both in longitudinal elastic modulus and in fatigue strength than those of a metallic material, therefore, it is quite possible that the composite material is appropriate to a prosthesis for hip joint broken due to a traffic accident or the like. However, the composite material has unavoidable anisotropy caused by woven cloth used as reinforcement, thus making it difficult to have the same isotropy as a metallic stem has. Yet, there is a possibility of obtaining pseudo-isotropy by alternately changing the direction of fiber of woven cloth, so that the method has been very well researched of introducing the composite material into artificial hip prosthesis stem.
A hip joint consists of a caput as a spherical joint which enables a personal body to rotate and bend by engaging with an acetabulum of pelvis and a neck connecting the caput with a femur. If the spherical joint is damaged, the hip joint loses its proper function. In order to recover it, the acetabulum is in need of repair, or as shown in FIG. 25 the pelvis and the neck are in need of replacement with a ball 31 and a stem 8 supporting the ball on the femur respectively as substitutes. The stem comprises a neck 1 supporting the ball in position and a body 2 fixing the neck to the femur.
The stem 8, when made of FRP, has a composite structure of, for example, woven cloth of carbon and a resin impregnated in the woven cloth, such as PEEK which is harmless to the human. The stem is formed by stacking the FRP materials in the cross section thereof with the materials concavely curved so as to fit the outer shape of the stem. More particularly, the stem is divided in the anterior and posterior direction of human body into two halves which mate together to form the stem. The divided surface 32 is selected so as to contain a longitudinal reference line 8a linking a center 1a on the end of the neck and a center 2a in the diaphysis of the distal portion of the stem. Each of the two halves is placed in a flat state to have an upper half 8U and a lower half 8L respectively. The FRP sheets are stacked in each of the molds where these upper and lower halves are formed. The upper half corresponds to the anterior portion of the stem and the lower half corresponds to the posterior portion of the stem (in the figure the stem is for a left leg). The stem 8 in the center of the figure is illustrated by using a set of lines of the edge of the prepreg sheets on the widest cross section thereof.
The prepreg sheets, i.e., thin sheets formed by impregnating woven cloth such as carbon with a thermoplastic resin, are heated to be deformable, for being overlaid in contact with the contour 33 of the cavity of the mold D, as shown in FIG. 26(a). In principle, the whole of the sheet 7i is covered with the sheet 7i+1 to be successively overlaid, thus multi-layers like growth rings of a tree may be formed in the mold. Applying the curing to the multi-layers in an autoclave enables the lower half 8L of the stem to obtain an outer shape that is identical to the cavity of the mold. The upper half of the stem which is formed in the same way as above, not shown, is stacked on the lower half, the two halves are contained in the mold to have the curing again, an FRP stem can be obtained whose neck 1 and body 2 are integrated. In U.S. Pat. No. 3,901,717 a method is disclosed that FRP is used for forming a stem.
With the stem 8 formed by curvedly stacking sheets like growth rings of a tree, air voids 34 are frequently produced on the cross section of the stem 8, as shown in FIG. 26(b). While the sheets are stacked, spaces 35 often remain between the sheets as shown in (c), which is an enlargement of section H of (a), but all the air cannot be squeezed out during the curing operation. On the Pascal's principle the pressure everywhere in the mold is uniformly maintained by the force F throughout the curing operation, and consequently it is impossible for the stem to have low-pressure area which enables release of the air, i.e., the sheets stick to each other at the both ends of the space where the air remains to form a closed area 35. The quality of stem depends mainly on an allowance for the quantity and the size of the air voids for producing the stem. Besides, in order to obtain pseudo-isotropy by stacking anisotropy sheets, it is necessary to alternately stack the sheets 7i having fibers arranged at angle of 0/90 degrees and the sheets 7i+i having fibers arranged at angle of ±45 degrees, however, which varies the direction of the plane rigidity every sheet, resulting in a decline in adherence of the sheets curvedly stacked.
In the field of optical molding, as disclosed in JP2001-347572A1, e.g., an desired shape of an object is obtained in the following method; contour lines of an object are calculated by level-cutting the three-dimensional data of the object, uncured photo-setting resin is sliced into some layers in uniform thickness on the basis of the contour lines, successively the layers are stacked to be integrated into one piece by curing.
When a three dimensional object is obtained by the optical molding, the primary purpose is to reproduce the outer shape of the object, not to obtain the structure so as to withstand complex loads, although it may be possible depending on the mechanical characteristics of cured resin. Even though a stem might be formed by using the method of optical molding, there is few possibility of obtaining a stem such that it can withstand tension, bending moment and shearing load caused by the influence of his weight or the like and has resistance to the load such as a hoop stress, what is more, these loads mentioned above will act irregularly. From this point of view, it is obvious that the FRP stacked product formed by applying the principle of optical molding thereto will not have the mechanical structure to be required as a stem.